Protective circuit for an electronic device

ABSTRACT

A protective circuit for an electronic device that is located between a constant voltage source (U v ) and the operating voltage input of the device (G) and consists of a diode (D V ) and a limiting resistor (R v ) connected in series. The diode (D v ) and the limiting resistor (R v ) are thereby bridged by a controlled switch (S) and a voltage monitor (W) is connected to an operating voltage (U B , U A ) of the device (G), said voltage monitor being devised to deliver a switch signal (s) that closes the switch as soon as a predeterminable minimum value of the operating voltage (U B ) has been reached.

[0001] The invention relates to a protective circuit for an electronic device that is located between a constant voltage source and the operating voltage input of the device and consists of a diode and a limiting resistor that are connected in series.

[0002] A plurality of electronic devices, such as switching power supply units for example, are fed from a constant voltage source, a battery for example, or from a local constant voltage network. When the device is connected to the constant voltage source with the wrong polarity, expensive component parts of the device may be destroyed as a result thereof so that a reverse battery protection is provided for in the form of a diode or a diode bridge for example. Due to inductive and/or capacitive component parts, the connection of the device to the constant voltage source may furthermore lead to very high transient current pulses, which also may cause the destruction of component parts. Although this may be prevented in many cases by fast blow fuses, e.g., safety fuses, it requires lots of work to replace the fuses; therefore, a limiting resistor, more specifically a thermistor, is used to limit inrush current.

[0003] In operation, such type protective circuits, consisting for example of a diode and a thermistor, produce permanent losses which reduce the overall efficiency of the device. When the device has an input current of 2A and when a diode and a 1 ohm limiting resistor are being used, the additional permanent loss amounts to approximately 5 Watt.

[0004] It is an object of the present invention to provide a protective circuit that will cause considerably less permanent losses.

[0005] The solution to this object is a protective circuit of the type mentioned herein above in which the diode and the limiting resistor are bridged by a controlled switch and a voltage monitor is connected to an operating voltage of the device, said voltage monitor being devised to deliver a switch signal that closes the switch as soon as a predeterminable minimum value of the operating voltage has been reached.

[0006] Thanks to the invention, the protective circuit is only in operation when it is actually needed, that is to say during transient. The permanent losses of the protective circuit are now only determined by the resistance of the closed switch and are up to two orders of magnitude below that of the protective circuits of the art.

[0007] Advantageously, the monitored operating voltage is the power supply voltage of the device.

[0008] If the device is a power supply unit, it may be practical that the monitored voltage be its output voltage.

[0009] A variant with very little permanent loss is characterized in that the controlled switch is constituted by a relay with a normally open contact. It may be advisable to connect a field winding of the relay in series with the collector-emitter-path of a gating transistor and to have said transistor gated by a voltage comparator of the voltage monitor.

[0010] If the device is a power supply unit, it is advisable to connect a field winding of the relay to the output voltage, the very relay including the voltage monitor.

[0011] In another variant the controlled switch is formed by a switching transistor. This permits to better control the switching operation. The switching transistor may for example be gated by a voltage comparator of the voltage monitor.

[0012] To limit the maximum input current, the switching transistor may be gated by a voltage comparator of the voltage monitor.

[0013] If a buffer resistor of low impedance is located in series with the controlled switch, disturbances in switching are kept particularly low although slightly higher losses have to be accepted.

[0014] With power supply units in particular, indirect coupling is advisable and may be achieved by connecting an optocoupler between voltage monitor and controlled switch.

[0015] The invention and all of its advantages will be apparent from the following more particular description of exemplary embodiments as illustrated in the drawing in which

[0016]FIG. 1 is a basic illustration of a protective circuit in accordance with the invention,

[0017]FIG. 2 is a view of a protective circuit in accordance with the invention, utilizing a relay

[0018]FIG. 3 is a view of a protective circuit in accordance with the invention, utilizing a switching transistor,

[0019]FIG. 4 is a view of a protective circuit in accordance with the invention for a power supply unit utilizing a relay, and

[0020]FIG. 5 is a view of a protective circuit in accordance with the invention for a power supply unit utilizing an optocoupler and a switching transistor.

[0021] As shown in FIG. 1, an electronic device G is intended to be fed from a constant voltage source U_(v). In the positive line, a protective diode D_(v) is located in series with a current limiting resistor R between the device G and the constant voltage source U_(v). This series connection D_(v)/R is bridged by the normally open contact of a controlled switch S, a voltage monitor W being provided for gating the switch. This voltage monitor W monitors an operating voltage U_(B) of the device, its input voltage for example, and is devised to supply a switch signal s to the controlled switch S as soon as an adjustable minimum value U_(M) of this operating voltage U_(B) has been reached.

[0022] In case the device G is connected to the constant voltage source U_(v) with a wrong polarity, no current can pass through the diode D_(v) so that the operating voltage will remain at zero or will never reach its predetermined minimum value, the switch S remaining open. If the device G is connected with proper polarity, a current flows through diode D_(v), said current being limited by limiting resistor R_(v) to such a value that the current surge that occurs is not inadmissibly high for neither the device G nor the constant voltage source U_(v). Such current surges are to be expected in particular when operating voltages are filtered by capacitors of high capacitance. Since sufficient current now passes into the electronic device G through limiting resistor R_(v), the monitored operating voltage U_(B) will increase as well and—as soon as it has reached its predetermined value U_(M)—the monitor W will deliver a switch signal s to the switch S which closes as a result thereof so that the voltage drops heretofore occurring at the diode D_(V) or at the limiting resistor R_(v) disappear and the corresponding power loss is no longer of any significance.

[0023] It should be noted at this point that the limiting resistor R_(v) may be dispensed with when the forward resistance of diode D_(V) has a value that is high enough for the respective one of the current limitation wanted.

[0024] In a practical realization according to FIG. 2, the monitoring circuit W is connected between a constant voltage U_(v) and the input operating voltage U_(B) of a device G. In the positive line, a diode D_(V) and a limiting resistor R_(v) are again located in series, this series connection being bridged by a normally open contact a of a relay A and a buffer resistor R_(v) being, in this exemplary embodiment, located in series with the normally open contact a. A comparator K is provided for monitoring the voltage. The positive input of this comparator K is supplied with the voltage, which is proportional to the operating voltage U_(B) of the device G, said voltage having been gathered from a voltage distributor R1/R2 and passed through a resistor R_(e) whereas the voltage at the negative input of the comparator K is the voltage of a Zener diode D_(Z) in the form of a reference voltage. The current passing through the Zener diode D_(Z) is produced by means of a resistor R_(Z). The output of the comparator K which is here provided with a resistor R_(r), is supplied to the base of a transistor T_(a) via a limiting resistor R_(a) in the collector circuit of which there is located the field winding A of relay A, a, whereas the emitter of transistor T_(a) is located at the negative line.

[0025] When switching the device G to the power supply voltage with the proper polarity, a current that is limited by the resistor R_(v) will flow into the device G, the operating voltage U_(B) of which will increase to the same extent as the voltage at the positive input of comparator K, and, as soon as a value is reached that exceeds the voltage of the Zener diode D_(Z), the comparator K will switch through, gate the transistor T_(a) and energize the coil of relay A, a. The normally open contact a closes and the operating voltage for the device G now passes through the buffer resistor R_(d). This buffer resistor is intended to prevent extreme voltage peaks and, as a result thereof, high-frequency disturbances, but in many cases it may be dispensed with.

[0026] If the constant current for the device G is assumed to be 2A and the contact resistance of the relay to be 10 m ohm the permanent power loss is of 40 m W. If, at the same current of 2A, the loss at the diode D_(v) is 0.7×2A, i.e., 1.4 W, and if a limiting resistor such as a thermistor with an operational resistance of 1 ohm is being used, the ohmic loss is of 4 W and the overall permanent power loss of 5.4 W. This power loss is two orders of magnitude higher than with the protective circuit according to the invention.

[0027] A variant of the invention utilizing a switching transistor T_(a) is shown in FIG. 3. In principle, this circuit resembles the one of FIG. 2 but for the relay contact that is replaced by the emitter-collector-path of the switching transistor T_(a), said switching transistor being supplied from the output of comparator K to the base of the switching transistor T_(a) via a resistor R_(t) and a dropping resistor R_(b), a resistor R_(c) being located in the collector of said transistor. As soon as, after connecting the device, the operating voltage U_(B) has reached a determined value that can be determined by the Zener diode D_(Z) or by the voltage distributor R1/R2 respectively, the switching transistor T_(a) is switched through and bridges the diode D_(v) and the dropping resistor R_(v). The resistor R_(c) again serves to prevent extreme current peaks. FIG. 3 also shows how for example a “slow” connection of the transistor may be achieved in order to additionally limit the maximum input current. The illustrated resistor R_(c) together with a capacitor C_(r5) serves this purpose, the switching delay and the rise of the current through transistor T_(s) respectively being determined by the time constant of the RC element R_(t)/C_(t). Of course, this RC element may also be dispensed with, the capacitor C_(t) being omitted and the resistor R_(t) being replaced by a short circuit in this case.

[0028] The embodiment according to FIG. 4 illustrates an exemplary embodiment in which the device is a power supply unit, e.g. a switching power supply unit, that is fed from a constant voltage source U_(v), an intermediate circuit voltage for example, and has an output constant voltage U_(A). In the positive input line of the device G, the diode D_(v) and the protective resistor R_(v) are connected in series in a manner well known in the art, this series connection being bridged by a normally open contact a of a relay A, a. The field winding A of the relay has, at the constant voltage output of the power supply unit G, a nominal output voltage U_(A). The relay, or the relay winding respectively, is dimensioned in such a manner that, when the output voltage U_(A) reaches a determined value, the relay responds, the contact a closes and the series connection R_(v)/D_(v) bridges. In addition to the low contact resistance of the normally open contact a, the use of a relay A, a also has the advantage of indirect coupling that may be necessary with a power supply unit for example.

[0029] Another variant of the invention according to FIG. 5 also illustrates an indirect coupling, although here, in a way similar to that shown in FIG. 3, a switching transistor T_(s) is being used. In the protective circuit according to FIG. 5, the output voltage of a power supply unit G is monitored and is supplied to the positive input of a comparator K via a voltage distributor R1/R2. At the negative input of comparator K there is the Zener voltage of a diode D_(Z) with a dropping resistor R_(Z) and the comparator K, which has a feedback resistor R_(r), gates the diode of an optocoupler O via a resistor R_(o). Collector C and emitter E of the transistor pertaining to the optocoupler are connected to the input side of the power supply unit G at the points labeled with the respective letters C and E and, when the optocoupler switches through, a base current flows through a resistor R_(b), the transistor of the optocoupler and into the base of the switching transistor T_(s) that now switches through. 

1. A protective circuit for an electronic device that is located between a constant voltage source (U_(v)) and the operating voltage input of the device (G) and consists of a diode (D_(v)) and a limiting resistor (R_(v)) connected in series, wherein the diode (D_(v)) and the limiting resistor (R_(v)) are bridged by a controlled switch (S) and a voltage monitor (W) is connected to an operating voltage (U_(B), U_(A)) of the device (G), said voltage monitor being devised to deliver a switch signal (s) that closes the switch as soon as a predeterminable minimum value (U_(M)) ) of the operating voltage (U_(B)) has been reached.
 2. The protective circuit of claim 1, wherein the monitored operating voltage is the power supply voltage (U_(B)) of the device (G).
 3. The protective circuit of claim 1, wherein the device (G) is a power supply unit and the monitored operating voltage its output voltage (U_(A)).
 4. The protective circuit of one of the claims 1 through 3, wherein the monitored switch (S) is formed by a relay (A, a) with a normally open contact (a).
 5. The protective circuit of claim 4, wherein a field winding (A) of relay (A, a) is located in series with the collector-emitter-path of a gating transistor (T_(s)) and said transistor is gated by a voltage comparator (K) of the voltage monitor (W) (FIG. 2).
 6. The protective circuit of claim 3 and claim 4, wherein a field winding (A) of the relay (A, a) is connected to the output voltage (U_(A)), the very relay including the voltage monitor (FIG. 4).
 7. The protective circuit of one of the claims 1 through 4, wherein the controlled switch is formed by a switching transistor (T_(s)) (FIG. 3).
 8. The protective circuit of claim 7, wherein the switching transistor (T_(s)) is gated by a voltage comparator (K) of the voltage monitor (W).
 9. The protective circuit of claim 8, wherein the output of the voltage comparator (K) is supplied to the switching transistor (T_(s)) via a delay element (R_(I), C_(I)).
 10. The protective circuit of one of the claims 1 through 9, wherein a buffer resistor of low impedance (R_(D)) is located in series with the controlled switch (S) (FIG. 2).
 11. The protective circuit of one of the claims 1 through 10, wherein an optocoupler (O) is connected between voltage monitor (W) and controlled switch (S). 